Liquid crystal display elements have been used for various electrical household machineries and equipment such as a clock and a calculator, measuring equipments, panels for automobile, word processors, electronic notes, printers, computers, televisions, and the like. Typical liquid crystal display systems include TN (twisted nematic) type, STN (super twisted nematic) type, DS (dynamic light scattering) type, GH (guest-host) type, IPS (in-plane switching) type, OCB (optically compensated bend) type, ECB (electrically controlled birefringence) type, VA (vertical alignment) type, CSH (color super homeotropic) type, FLC (ferroelectric liquid crystal), and the like. Moreover, regarding a driving system, multiplex driving system has become typical from conventional static driving system, and simple matrix systems and recently active matrix (hereunder, referred to as “AM”) systems in which driving is performed by TFT (thin film transistor), TFD (thin film diode), and the like, have been in the mainstream.
In these display systems, IPS type, ECB type, VA type, CSH type, and the like are different from TN type and STN type which are currently used for general purpose, and are characterized in that liquid crystal materials having a negative dielectric anisotropy (Δ∈) are used. Of these, in particular, VA type display by means of AM driving is the most expected type at present in the application to display elements that require high-speed and wide-field angle, such as a television.
For liquid crystal materials used for display systems such as VA type, a low-voltage driving property, quick responsiveness, and a wide operating temperature range are required. That is, a negative dielectric anisotropy of a large absolute value, a low viscosity, and a high nematic phase-isotropic liquid phase transition temperature (Tni) are required. Therefore, in order to realize quick responsiveness, an attempt has been made to reduce the cell gap of display elements. However, in order to optimally set the retardation represented by the product (Δn×d) of the refractive index anisotropy (Δn) and the cell gap (d), the refractive index anisotropy and the cell gap of a liquid crystal material have to be adjusted within appropriate ranges. Accordingly, narrowing of the cell gap has been limited. In order to improve the response speed without changing the cell gap, it is effective to use a liquid crystal composition with a low viscosity. If a liquid crystal display element is applied to a television or the like, since quick responsiveness is prioritized, the development of liquid crystal compositions having a low viscosity has been particularly in demand.
As a liquid crystal material having a negative dielectric anisotropy, liquid crystal compounds having a 2,3-difluorophenylene structure as follows are disclosed (refer to Patent Documents 1 and 2).
(wherein R and R′ represent an alkyl group or an alkoxy group of 1 to 10 carbon atoms).
Furthermore, in these cited documents, compounds having a 1-hydroxy-2,3-difluoro-4-substituted benzene structure are disclosed. However, the compounds described in the cited documents are comprehensive in a wide range, and there is no specific disclosure of a compound having alkenyl groups at the both side chains. With liquid crystal compositions having a negative dielectric anisotropy using the described compounds, a sufficiently low viscosity has not been realized yet in liquid crystal compositions in which quick responsiveness is required for a liquid crystal television or the like.
On the other hand, there are disclosures of liquid crystal compositions using compounds having a 1-hydroxy-2,3-difluoro-4-substituted benzene structure serving as a basic structure of a liquid crystal compound constituting the invention of the present invention (refer to Patent Documents 3, 4, and 5). However there is no specific disclosure of a liquid crystal composition using a compound having alkenyl groups at the both side chains, and there is no specific disclosure of the type of compound to be used in addition to the concerned compound so as to reduce the viscosity of the liquid crystal composition.
Moreover, liquid crystal compounds having a 2,3-difluorohydroquinone structure have been already disclosed (refer to Patent Documents 6 and 7), and liquid crystal compositions using the concerned compound have been also disclosed. However, the concerned compound has a hydroquinone structure, and thus is considered to be unusable for an active matrix in the point of the voltage-holding ratio (refer to Non Patent Document 1), delaying the development of a liquid crystal composition having a low viscosity for VA using the concerned compound.
Liquid crystal compounds having 7,8-difluorochroman structure has been disclosed as a liquid crystal compound having a negative dielectric anisotropy of a large absolute value (refer to Patent Document 8). The liquid crystal compounds and the liquid crystal compositions thereof have a negative dielectric anisotropy of a large absolute value, but a viscosity is not sufficiently low. Accordingly, the development of a liquid crystal compound and a liquid crystal composition with a low viscosity is desired.
As a liquid crystal compound with a low viscosity, there is a compound having an alkenyl group as a side-chain, and many compounds have been already disclosed (refer to Patent Document 9). However, in this document, liquid crystal compounds and liquid crystal compositions having a positive dielectric anisotropy are taken in consideration, and there is no detailed disclosure on the following: what kind of compound is specifically used from among a lot of disclosed compounds for a liquid crystal composition having a negative dielectric anisotropy; what kind of compound is used together; and how the effect of the combination of compounds is.
Accordingly, the development of a liquid crystal composition having a negative dielectric anisotropy with a low viscosity is desired.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. S60-199840    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H2-4725    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. H8-104869    [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2000-96055    [Patent Document 5] European Patent Application, Publication No. 0474062 (page 14)    [Patent Document 6] Published Japanese Translation No. H2-503568 of the PCT International Publication    [Patent Document 7] German Patent Application, Publication No. 3906058    [Patent Document 8] PCT International Publication No. WO2005/000995    [Patent Document 9] Japanese Unexamined Patent Application, First Publication No. H10-45639    [Non Patent Document 1] Hiroshi Numata, Monthly Display, Vol. 4, No. 3, pp. 1-7, (1998) (page 5, Table 4)